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Full text of "The Cambridge Natural History"

16                   MARSUPIUM   AND   MAMMARY   POUCH                CHAP.
supials.1 It is usually held that this structure is not of pre-
cisely the same morphological value as the pouch of the
Marsupial; and the difference is expressed by terming the one
(that of Echidna} the mammary pouch, and the other the
marsupium. At first sight it may appear to be an unnecessary
refinement to separate two structures which have so many and
such obvious likenesses. It is not quite certain, however, that the
difference is not even more profound than later opinions seem
to indicate. The Monotremata not only have no teats, as has
already been pointed out, but the mammary glands themselves
are of a perfectly different nature to those of the higher mammals,
including the Marsupials. Tiiere is therefore no a priori
objection to the view that the accessory parts developed in con-
nexion with the mammary glands should also be different. The
teat of the higher Mammalia grows up round the area upon
which the ducts of the mammary glands open; it is a fold of
skin which eventually assumes the cylindrical form of the <uhilt
teat, and which includes the ducts of the milk glands. It has
been suggested that the two folds of skin which form the
mammary pouch of Echidna are to be looked upon as the equi-
valent of the commencing teat of the higher mammal,2 In this
case it is clear that the marsupial folds of the Marsupial cannot
correspond accurately with the apparently similar folds of
Echidna, because there are teats as well. It is the teats which
correspond to the marsupial folds of Echidna. This view is in
apparent contradiction to an interesting discovery in a specimen
of a Phalaixgcr by Dr. Klaatsch.3 This Marsupial, like most
others, has a well - developed marsupial pouch, in which the
young are lodged at birth; but round two of the teats "is
another distinct fold on either side, the outer wall of which
forms the general wall of the pouch. Dr. Klaatsch thinks
that these smaller and included pouches are the equivalents of
the mammary pouchos of Echidna. They contain teats, but this
comparison does not do away with the validity of Gegeiibaur's
suggestion already referred to, because the teats are (see above)
1 Sec Haacke, " On the Marsupial Ovum, tlio JVlairumary IVu01i, etc., of tho
Echidna," Proc. Roy. SOG. 1885, p. 72 ; and " Uber die Eutatohtmgder S&ugetiere,'*
Biol. Oentralbl. viii. 1889, p. 8.
a See Gegenbaur's Elements of Cvmp. Anat.    Transl. by Bell, 1878, p. 421.
8 "thber die Buzielximgen zwiscliou Mamwiartusclie u. Marstipitmi," Morph,
Jahrb. xviL 1891, p. 483.